Difficult as it is to believe now, there was a time when Robert Mugabe was widely thought of as a "great, great man". A man who brought peace to newly liberated Zimbabwe after 15 years of war, who was awarded an honorary doctorate from Edinburgh University (since revoked), and who has now, in the words of Jason Wallace's bleak, ferocious Costa award-winning debut Out of Shadows, "run this forsaken country into the ground . . . a country [that] is burning, yet no one will tackle the flames".

It's 1983 in suburban Harare, only recently renamed from Salisbury. Robert Jacklin and his mother have moved from England so his well-meaning but ineffective father can take up a civil service post. Both Robert and his mother opposed the move, she taking to her bed with drink and Robert shunted off to Haven, a mostly white boarding school just starting to ring the changes of this new, no longer minority-ruled country.

At first, Out of Shadows seems to be setting itself up as a predictable white-man-in-Africa story. Robert's father insists that he become friends with one of the school's few black pupils, the somewhat weightily named Nelson, who is perhaps a bit too saintly. Robert and Nelson bond quickly, though, promising to watch out for each other like brothers, and particularly to avoid the attentions of white, racist bully Ivan, who seethes that "his" country has been unfairly lost.

Then the story takes an unexpected – but entirely convincing – turn. Robert's friendship with Nelson doesn't last very long, certainly not when Ivan starts to take a fraternal interest in Robert. This isn't necessarily an entirely self-interested decision on Robert's part – though as a "Pom", the protection of Ivan would be useful – but Ivan embodies a rageful, wounded certainty that starts to appeal, dangerously, to Robert, who in his own way has also lost his home. Soon enough, Robert is spending weekends at Ivan's family farm, run by his furious, abusive father. We begin, slowly, to understand where Ivan's anger comes from. Wallace spares us nothing in his depiction of the brutality of the war that has just ended – one of Ivan's cohorts had a brother who was found "pinned to a tree with his own cock in his throat" – or in the inevitable fate of white farmers in Zimbabwe.

Wallace is playing a risky game here. He leads his reader into the mindset of a resentful white Zimbabwean so convincingly that we are in danger of sympathising with it, especially with what we know of Mugabe's future actions. Such is the seductive appeal of that anger and certainty that Robert himself is deeply, bloodily complicit before he realises he's become the very bully he used to despise. Can he stop Ivan's final, ambitious plan, or is it too late to save even himself?

Out of Shadows isn't perfect – the plotting is often unsubtle and occasionally unconvincing, particularly a final swerve into thriller territory; adults are implausibly absent at almost all stages; and the full psychology of Robert's seduction into and journey out of Ivan's brand of hatred isn't as nuanced as it really needs to be – but there is terrible power here. Wallace has paid his readers the compliment of not looking away, of reporting what he sees as he sees it, which is what any artist does.

Even for an age group that prizes bleakness, this is a harrowing read, with no punches pulled on excessive profanity or shocking violence. This is certainly not a book for younger readers, but even with its rough edges, it's a powerful, devastating read for older teens.